The variations of the selectivity coefficient K-A(B) between Na+-H+, Na+-K+, and Na+-Cu2+ systems and the separation factor alpha(A)(B) between Na+-Cu2+ and K+-Cu2+ systems in cation-exchange membranes as functions of loading and particle size of resin have been measured. The exchange affinities of all the membranes increase as H+ < Na+ < K+ < Cu2+. K-A(B) is almost constant, independent of particle size and loading of resin in the membrane. alpha(Na)(Cu) and alpha(K)(Cu) increase with decrease in particle size but remain almost constant with loading of resin in the membranes. The order of selectivity is K-Na(H) < K-Na(K) < K-Na(Cu). The values of (alpha(A)(B)Q(0)(-1))other(alpha(A)(B)Q(0)(-1))(M(300)C60)(-1) for the Na+-Cu2+ system are approximately 1.060 and for K+-Cu2+ are approximately 0.870. Q(0) represents the exchange capacity of a dry membrane per unit volume (in milliequivalents per cubic centimeter).